Thirteen New Instructions - SSE3

Back at IDF we learned about the thirteen new instructions that Prescott would bring to the world; although they were only referred to as the Prescott New Instructions (PNI) back then, it wasn't tough to guess that their marketing name would be SSE3.

The new instructions are as follows:

FISTTP, ADDSUBPS, ADDSUBPD, MOVSLDUP, MOVSHDUP, MOVDDUP, LDDQU, HADDPS, HSUBPS, HADDPD, HSUBPD, MONITOR, MWAIT

The instructions can be grouped into the following categories:

x87 to integer conversion
Complex arithmetic
Video Encoding
Graphics
Thread synchronization

You have to keep in mind that unlike the other Prescott enhancements we've mentioned today, these instructions do require updated software to take advantage of. Applications will either have to be recompiled or patched with these instructions in mind. With that said, let's get to highlighting what some of these instructions do.

The FISTTP instruction is useful in x87 floating point to integer conversion, which is an instruction that will be used by applications that are not using SSE for their floating point math.

The ADDSUBPS, ADDSUBPD, MOVSLDUP, MOVSHDUP and MOVDDUP instructions are all grouped into the realm of "complex arithmetic" instructions. These instructions are mostly designed to reduce latencies in carrying out some of these complex arithmetic instructions. One example are the move instructions, which are useful in loading a value into a register and adding it to other registers. The remaining complex arithmetic instructions are particularly useful in Fourier Transforms and convolution operations - particularly common in any sort of signal processing (e.g. audio editing) or heavy frequency calculations (e.g. voice recognition).

The LDDQU instruction is one Intel is particularly proud of as it helps accelerate video encoding and it is implemented in the DivX 5.1.1 codec. More information on how it is used can be found in Intel's developer documentation here.

In response to developer requests Intel has included the following instructions for 3D programs (e.g. games): haddps, hsubps, haddpd, hsubpd. Intel told us that developers are more than happy with these instructions, but just to make sure we asked our good friend Tim Sweeney - Founder and Lead Developer of Epic Games Inc (the creators of Unreal, Unreal Tournament, Unreal Tournament 2003 and 2004). Here's what he had to say:

Most 3D programmers been requesting a dot product instruction (similar to the shader assembly language dp4 instruction) ever since the first SSE spec was sent around, and the HADDP is piece of a dot product operation: a pmul followed by two haddp's is a dot product.

This isn't exactly the instruction developers have been asking for, but it allows for performing a dot product in fewer instructions than was possible in the previous SSE versions. Intel's approach with HADDP and most of SSE in general is more rigorous than the shader assembly language instructions. For example, HADDP is precisely defined relative to the IEEE 754 floating-point spec, whereas dp4 leaves undefined the order of addition and the rounding points of the components additions, so different hardware implementing dp4 might return different results for the same operation, whereas that can't happen with HADDP.

As far as where these instructions are used, Tim had the following to say:

Dot products are a fundamental operation in any sort of 3D programming scenario, such as BSP traversal, view frustum tests, etc. So it's going to be a measurable performance component of any CPU algorithm doing scene traversal, collision detection, etc.

The HSUBP ops are just HADDP ops with the second argument's sign reversed (sign-reversal is a free operation on floating-point values). It's natural to support a subtract operation wherever one supports an add.

So the instructions are useful and will lead to performance improvements in games that do take advantage of them down the road. The instructions aren't everything developers have wanted, but it's good to see that Intel is paying attention to the game development community, which is something they have done a poor job of doing in the past.

Finally we have the two thread synchronization instructions - monitor and mwait. These two instructions work hand in hand to improve Hyper Threading performance. The instructions work by determining whether a thread being sent to the core is the OS' idle thread or other non-productive threads generated by device drivers and then instructing the core to worry about those threads after working on whatever more useful thread it is working on at the time. Unfortunately monitor and mwait will both require OS support to be used, meaning that we will either be waiting for Longhorn or the next Service Pack of Windows for these two instructions.

Intel would not confirm whether the instructions can be used in a simple service pack update; they simply indicated that they were working with Microsoft of including support for them. We'd assume that they would be a bit more excited about the ability to bring the instructions to Prescott users via a simple service pack update, maybe indicating that we will have to wait for the next version of Windows before seeing these two in use.

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  • TrogdorJW - Tuesday, February 3, 2004 - link

    Technically, it depends on how you cound pipelines. The P4 has several "simple" pipelines that deal with the easy instructions, and then "complex" pipelines that deal with the more difficult instructions.

    For example, they have two Integer units running at twice the core clock speed, but those only do simple integer instructions. Then they have a complex Integer unit running at core speed that can do the remaining integer instructions. So that's 3 INT units, technically, and two of those are double-pumped, so you could even call it five INT units if you want to be generous.

    The FP/SSE is somewhat similar, I believe. The end result is that it's not an apples-to-apples comparison between Intel and AMD pipelines. You could really say both of them have nine different execution units (pipelines), but Intel's pipelines aren't as powerful as AMD's when compared directly. See: http://www.tomshardware.com/cpu/20040201/prescott-... - there is an image of the pipelines in the Prescott, which is mostly unchanged from the Northwood.

    The thing with the number of stages in a pipeline still holds true. So you have 60 million transistors in 7 pipelines, each with 31 stages. (Actually, the FP pipelines probably have more stages.) That still gives you a rough guess of 275000 transistors in each pipeline stage. In the P4, it was 30 million transistors in 20 stages and still 7 pipelines, giving a guess of 215000 transistors per stage.

    I'm really, REALLY curious as to what Intel is doing. For some reason, the core of the P4 in the Prescott is at least twice as big (in transistor count) as the core of the Northwood. The L2 cache is also twice as big. So we went from 29+26 million transistors in Northwood (core+L2 cache) to apparently something like 75+50 in the Prescott.

    If indeed there are 75 million transistors in the Prescott core, they *had* to increase the length of the pipelines to 30 or so stages to have any chance of running fast. However, you can't argue that the increase in transistors was necessitated by the increase in the number of pipeline stages! Why? Apparently, the Prescott has more transistors per stage, so in theory a Northwood would have actually scaled to *higher* clockspeeds than a Prescott!

    Intel is definitely not showing all of their cards on the table right now. I'm betting that they're trying to protect Itanium as long as they can. I guess we'll know sometime in the next year or so.
    Reply
  • KristopherKubicki - Tuesday, February 3, 2004 - link

    Check out Anand's Blog on x86-64 for Intel

    http://www.anandtech.com/weblog/index.html?bid=46

    Kristopher
    Reply
  • Pumpkinierre - Tuesday, February 3, 2004 - link

    Errata for #87 2nd paragraph: 'According to Ace's'- not Ace's but X-bit:

    http://www.xbitlabs.com/articles/cpu/display/presc...

    Thank you #89 although I didnt think the P4 had as many pipelines as you quote.
    Reply
  • INTC - Tuesday, February 3, 2004 - link

    http://69.56.255.194/?article=13959

    Hmmm - wouldn't that be exciting? P4 Prescott 3.2E GHz with XDR Rambus at 3.2 GHz PCI Express and 64-bit extensions at IDF - I wonder when Nventiv will have that in their new Cold Fusion systems?
    Reply
  • DerekBaker - Tuesday, February 3, 2004 - link

    On the most common way of counting the Athlon has 9 pipelines, the P4 7.


    Derek
    Reply
  • Pumpkinierre - Tuesday, February 3, 2004 - link

    Add "So to compensate the slower speed of shorter pipelines, they make them more numerous in a cpu eg 6-8 in Athlons cf. 3 in P4 (I believe)" to the middle of 1st paragraph Reply
  • Pumpkinierre - Tuesday, February 3, 2004 - link

    #82 There is more than one pipeline in a processor so you have to take that into account in your stage/No. of transistor calculations plus registers, buffers, stacks, MMX, SSE etc.. I also am not totally happy with the AT explanation of pipelines. Pipelines are just a way of guessing the correct answer so that idle cpu time can be put to good use. I thought the stages in a pipeline be it 10 or 20 were of the same complexity. Its just that the outcome of a longer pipeline had a lower probability of being correct due to the increased likelihood of more branch statements being present in a longer pipeline. But work in checking the correct outcome is less in a longer pipeline. Work is heat so smaller pipelines make more heat which lessens speed headroom while longer pipelines can run at higher speed but correct outcomes are less probable. So to compensate they use more pipelines. Paradoxically with Prescott they've increased the pipeline lenght but they have more heat so as far as I am concerned speed headroom is limited and I doubt they will get past 4Gig with the present cpu. The o'clocks so far bear this out, with stable bests at ~3.8GHz. This is as result of some physical problem with the 90 nm process. What they should have done is applied the tweaks to the Northwood 130nm core and they would have been heaps better off. Its doubtful whether the tweaks would have increased temperature but they would be getting 30 to 50% better calculating power from the cpu at the same core speed. Would'nt need to PR rate it, just call it a different name. Then they would have had more time to sort out the 90nm problem while keeping the consumer happy. As it is they are going to cop a lot of flak over this overbaked failure.

    I'm also not happy about this loss of latency in the caches. Even though i've abused large caches in the past, that was on the grounds of gaming software where i expected alot of cache misses by the cpu because of the unpredictable nature of operator driven gaming. But here they are saying the latency has increased (and tests measure this) no matter the application and the reason given by sites is the doubling in size of the cache. But when the P4 went from 256K L2 to 512k L2 and the A-XP(256K) to Barton(512K) or even A64 3000+(512K) and 3200+(1024K) no major increase in cache latency was reported- in fact often the opposite. According to Ace's the latency of the Prescott 16K data L1 cache is now close to that of the a64 L1 (64K data) 4 times its size and double the latency of the Northwood 9even though Intel says it is the same- but no figures)! Something weird's going on with this 90nm stuff.

    Reply
  • PrinceGaz - Tuesday, February 3, 2004 - link

    Hmmm... where to begin :)

    Okay, first of all I must say that was an excellent review overall and the background material covering all the architectural changes was nothing short of superb. I'll definitely re-read chunks of that whenever I need a refresher on various aspects of its design.

    Your overclocking results were very good, far better than those achieved by most other sites. However I think it was a bad idea for AnandTech to suggest a Prescott is a great overclocker based on the sample(s) they received from Intel. It would be better to wait until you've got some retail CPUs from other sources before making recommendations about buying it for overclocking as readers may not be so lucky as you were.

    Right, onto the tests... overall as I see it the Prescott is really pretty much on a par with the Northwood performance-wise for a given clock-speed. Its faster at some tasks by a small margin thats not significant, and slower at as many others by a similar small margin I wouldn't worry about. As such it won't matter to an average user whether they get a P4 3.4C or a P4 3.4E processor. Therefore everything that has been said comparing the Northwood to the A64 is still valid when comparing the Prescott to the A64 (at least at clock-speeds over 3GHz).

    As many others have commented, the omission of any mention at all of the thermal issues was nothing short of staggering. *Every* other major review I read at least said something about it and most of them had quite a lot to say about it. I did notice the occassional error in what they said such as at [H]ard where their Prescott was running at 1.5V which therefore invalidated their temperature readings but even on those sites where it was running at the correct voltage, heat was still an issue.

    Its quite possible the current version of the Prescott is a bit like AMD's first 130nm chip the Thoroughbred 'A' which also ran rather hot. Of course this is already supposed to be the third revision of the Prescott so whether they can make any further tweaks that will seriously reduce power requirements is debatable. If they can't then ramping up the speed up to 4GHz and beyond that in 2005 will be a major problem. The most conservative estimate based on current figures would be for a 4GHz chip to have a TDP of 130W though in reality thats likely to be closer to 150W. Even if improved cooling solutions are able to get rid of that much heat from the chip *and* the case, electricity isn't free so the cost of running it must be considered to.

    Finally about 64-bit support in the Prescott. It wouldn't surprise me if Prescott does have 64-bit support built into it which is currently disabled in much the same way Hyper-Threading was disabled in some Northwood cores. The only people who know for sure either work for Intel and arent saying, or are under NDA. It would be a blow to IA64 (and also in a way be seen as saying AMD was right) if Intel did suddenly enable x86-64 support so I doubt they'll do so unless the case becomes compelling. Theres no sign of that happening in the immediate future.
    Reply
  • KristopherKubicki - Tuesday, February 3, 2004 - link

    They put 30M extra transistors on there to confuse people. :(

    Kristopher
    Reply
  • TrogdorJW - Tuesday, February 3, 2004 - link

    Actually, Icewind, if they don't *have* to activate the 64-bit capability, then they're okay. I mean, activating 64-bit in x86 is basically the death toll for Itanium and IA-64. That would make some (*all*?) of the companies that have purchased and worked on IA-64 rather pissed, right?

    If Prescott does have 64-bit, it was just Intel hedging their bets. They would have started design on the new core 2 years ago, around the time when the full specifications of AMD64 were released. Intel couldn't know for sure what the final result of K8 would be, so they may have decided to start early, just in case.

    Like I said before, it's pure speculation at this point, but I figure adding 64-bit registers and instructions to x86 could be done with 10 to 15 million transistors "easily". I've basically figured out (as others have, apparently) that there are close to 30 million transistors that aren't accounted for in the Prescott. That's the size of the entire Northwood core (minus cache)! If you have a better idea of where these transistors were used, feel free to share it. :)
    Reply

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